In electronic devices, high-density mounting of components is usually implemented by mounting sub circuit boards on printed circuit boards that are arranged in a casing or a chassis. Electronic components, which generate heat when they are operated, are mounted on the printed circuit boards and the sub circuit boards. An increase in temperature of an electronic device due to heat generated by the electronic components causes the electronic device to operate abnormally. Accordingly, in electronic devices, electronic components that generate heat need to be cooled.
With conventional technology, when a sub circuit board is mounted on a printed circuit board that is arranged in a casing of an electronic device, multiple riser units in which a sub circuit board is mounted on a riser card are usually mounted on the printed circuit board. Furthermore, as illustrated in FIG. 11A and FIG. 11B, on a system board 20, a riser unit U1 and a riser unit U2 are usually mounted on a main circuit board 12 such that a section board 9 of the riser unit U1 and a section board 7 of the riser unit U2 are aligned in the same plane as that of the front surface S, which is perpendicular to a main circuit board 12.
The electronic components mounted on the sub circuit board on the riser unit U1 are cooled by cooling air flowing from ventilating holes included in the section board 9, whereas the electronic components mounted on the sub circuit board on the riser unit U2 are cooled by cooling air flowing from vent holes and ventilating holes 22 included in the section board 7.
Furthermore, there is a known technology in which, when multiple printed circuit boards having mounted thereon electronic components in a casing of the electronic device are mounted in parallel in the casing of the electronic device, the multiple printed circuit boards are mounted by shifting, from among the multiple printed circuit boards, the arrangement position of one printed circuit board forward or backward with respect to the other printed circuit board. Furthermore, there is also a known technology for forming a single casing by dividing a casing of an electronic device into two, by shifting one divided casing in the vertical direction with respect to the other divided casing, and by combining the two divided casings.
Then, air intake and discharge holes, through which cooling air flows that cools the electronic components, are arranged at a portion created by shifting the position of the printed circuit board forward or backward with respect to the other printed circuit board. Alternatively, the air intake and discharge holes are arranged at a portion created by shifting one part of a divided casing in the vertical direction with respect to the other part of the divided casing and by combining the two divided casings. As described above, with the conventional technology, the efficiency with which the electronic components mounted on the printed circuit board are cooled may be improved by increasing the number of air intake and discharge holes and by causing a larger amount of cooling air to flow over the printed circuit board arranged in the casing.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2002-366258
Patent Literature 2: Japanese Laid-open Patent Publication No. 2008-251083
However, with the conventional technology described above, because the number of air intake ports is insufficient, there is a problem in that the cooling efficiency of the entire system board 20 including the electronic components mounted on the sub circuit board on the riser unit is low.
Specifically, as illustrated in FIG. 11A, the electronic components on the sub circuit board on the riser unit U2 are cooled by the cooling air flowing from the vent holes and the ventilating holes 22 that are included in the section board 7. However, the electronic components on the sub circuit board on the riser unit U1 are cooled only by the cooling air flowing from the vent holes that are included in the section board 9. Accordingly, with the conventional technology, because the amount of cooling air flowing over the system board 20 is insufficient, the cooling efficiency of the entire system board 20 including the electronic components mounted on each of the sub circuit boards of the riser unit U1 and the riser unit U2 is low.